Aluminum wiring film formed by a sputtering method has been widely used in semiconductor integrated circuits and flat panel displays due to its low resistivity, good etchability, and low manufacturing cost. Low resistivity and high thermal conductivity lead to low resistance-capacitance (R-C) delay associated with the interconnection network. R-C delay is a critical factor in determining the signal propagation speed or the time constant in the devices and circuits. For example, it is necessary to maintain a low time constant and keep an electrical resistivity below 5 μΩ·cm and even below 3 μΩ·cm for the wiring films connecting the sources and drains of the amorphous thin film transistors (TFT) of liquid crystal displays (LCD) to sustain desirable display quality and power consumption when the size of the display panel becomes large.
For many applications it is critical that the wiring film is of uniform thickness over the entire deposited substrate. This is especially true for large-scale integrated circuits consisting of multiple layers of multilevel structure having feature size of 1 micrometer or less. The production of a single multilevel structure involves several sputtering and patterning process including depositing and patterning dielectric material, depositing a diffusion barrier layer, and depositing and patterning a conductive wiring film. The variation in wiring film thickness not only causes inconsistent signal propagation speed and power consumption due to the varied film sheet resistance (Rs), which is inversely proportional to the film thickness, but also adversely affects the performance of the layers built on the wiring film or even causes short circuits between the conductive wire films as a result of the formation of large film bumps-hillocks.
The thickness uniformity of wiring films is believed to be directly influenced by the structural characteristics of the sputtering target including grain size, orientation, and the uniformity of their distribution. The target grain structure is typically controlled through controlling its fabrication process consisting of mechanical deformation and thermal anneals. A key step to form desirable target grain structure is to accumulate sufficient and uniformly distributed internal energy in the deformation process (roll, press, forge, extrusion or their combination). The internal energy is the driving force for the grain refinement in the recrystallization anneal process. However, it has been observed that high purity aluminum (5N or higher purity) can undergo a dynamic recrystallization during a hot deformation. One of the consequences of the dynamic recrystallization is that the internal energy is partially lost. The grain refinement process in the subsequent static recrystallization process can be incomplete or never happen due to insufficient internal energy. The other consequence of the dynamic recrystallization is the formation of non strain-free recrystallization grains dispersed in the deformed matrix of high dislocation density. This kind of nonuniform partial recrystallization structure results in considerable variations in the thickness or flatness characteristics of the deposited films because the recrystallization grains and deformed matrix have different sputtering behaviors.
An issue associated with the applications of pure aluminum film is its low electromigration resistance and thermal stability. Many aluminum wiring film failures are caused by the electromigration which occurs and leads to a directional mass transport associated with atomic flux divergence when the wiring film is subjected to high current densities. Voids or hillocks form in the films of low thermal stability subjected to a thermal treatment or a joule heat generated by a high current density. In general, the electromigration resistance increases with increasing thermal stability. A common solution to enhance the thermal stability and electromigration is to alloy the aluminum. Adding up to 0.1 wt % Cu, Fe, Ti, and B alloying elements to the pure aluminum target has been reported to improve the thermal stability of the deposited films. However, alloying aluminum with impurity elements can increase the electrical resistivity of aluminum. On the other hand, adding alloying impurities to aluminum degrades the etchability of aluminum. The commonly used Al alloying element Cu can deteriorate the patternability of Al because the Cu and Al can form very stable intermetallic precipitates which are difficult to be removed by Al etching reactant, and the etching reactant suitable for Al will react with Cu to form compounds that are insoluble in the commonly used cleaning solvents.
Accordingly, there is an ever-increasing demand to develop an aluminum or aluminum alloy target resulting in wiring films with improved uniformity, electromigration resistance, and thermal stability while maintaining low resistivity and good etchability to meet the needs of current and future in semiconductor electronic devices and flat panel display applications.